This patch implements basic batched processing of tx virtqueue byprefetching desc indices and updating used ring in a batch. Fornon-zerocopy case, vq->heads were used for storing the prefetchedindices and updating used ring. It is also a requirement for doingmore batching on top. For zerocopy case and for simplicity, batchedprocessing were simply disabled by only fetching and processing onedescriptor at a time, this could be optimized in the future.